I think it is more you start out trying to fix the worlds problems, and then after some years you give up and become happier. I don't think it is wisdom or maturity, many people give up as a kid and just go through the motions from the start, I wouldn't call those wise or mature.
> I think it is more you start out trying to fix the worlds problems,
I can't think of that many young twenty-somethings who want to fix the worlds problems. If anything, younger people are more focused on themselves [1] that people after the 30-35 yo mark.
[1] That's not an accusation or anything. It's natural that, when you're young, you need to figure out who you are and what you want out of life, and that requires a lot of focusing on oneself. Not to mention just growing into being an actual adult is also not easy and requires a lot of trial, error and introspection - something that is not as required when one is older and more formed already.
I think both generalizations are off the mark a bit. A lot of young folks are very idealistic and think they can have a larger impact (i.e. fix the world's problems). You also have those that are self-focused. Some of that is combined - focused on self means you may have an over-inflated sense of what you can accomplish and how your contributions will be received.
A lot of that is why some young entrepreneurs do go on to actually change the world (and we definitely need those people, although it isn't always for the best). But most that think that's what they are going to do fail a few times, and become more realistic in their views of what they can accomplish.
Sometimes the smarter someone is, the more quickly they can think through the possible results and come to those conclusions that they probably aren't going to be one of the few that does succeed to change the world, and that can (to the overall point here) lead to a sense of sadness and loss, maybe without having gone through the effort and failure, even. Which is a bit sad for everyone, because some percentage of those people probably could succeed if they just got out of their own way and stopped over thinking things, also.
Speaking as someone not totally young, and (hopefully) not totally un-smart, I think its true one mellows out with age, also you can "be the change you want to see". That to me is the only thing we can do. If we all do our bit we can make things a bit better. Some people can make a disproportionately large positive difference. Its never too late to do that.
